How do I keep Word document formatting the same as it appears in Word when copy/pasting into InDesign?
I have tried several methods and the text becomes homogenized in InDesign and loses all formatting, including bold and italics. This will be for formatting an ad journal of 300-500 pages.
Another option from the one mentioned in the comment: You can link your Word file the same way as you do for images and even use the same stylesheet as Word if you want. If you work on a big file, it can be useful and even faster than always copy/pasting texts.
When you import your text file, you can select to preserve some styles, images and formatting. You can select "preserve styles and formatting from text and tables."
If you had stylesheets in Words, you might need to merge them with your InDesign ones or replace them. When you'll make updates on the text of your Word file, it should update as well in your InDesign file but it doesn't work the other way around (from InDesign to Word), so be careful! It works in the same way as imported vector or raster work in InDesign.
Even if InDesign allows inline graphics import from the Word file, it doesn't always work well. But the technique is perfect for text-only Word file.
Here's a link with some details about it preserving stylesheets: